The mechanism by which specificity of physiological responses are conferred by a limited number of signal transducing substances, typically enzymes, is poorly understood. Cellular receptors on the surfaces of various cells are involved and initiate multiple signaling pathways. Some of the receptors on neutrophils are known: the PAF receptor, the interleukin-8 receptor and the fMetLeuPhe receptor all belong to the super-family of G-protein-linked receptors. A common feature of these receptors is that they span the cell membrane seven times, forming three extracellular and three intracellular loops and a cytoplasmic carboxy-terminal tail. The third loop and the tail exhibit extensive variability in length and sequence, leading to speculation that these parts are responsible for the selective interaction with the various G-proteins. Many of these G-protein-linked receptors stimulate the activation of three phospholipases, phospholipase C (PLC), phospholipase D (PLD) and phospholipase A.sub.2 (PLA.sub.2). These phospholipases constitute a family of regulatory enzymes which trigger various neutrophilic functions, for example adherence, aggregation, chemotaxis, exocytosis of secretory granules and activation of NADPH oxidase, i.e., the respiratory burst.
The main substrates for the phospholipases are membrane phospholipids. The primary substrates for PLC are the inositol containing lipids, specifically and typically phosphotidylinositol (PI). PI is phosphorylated by PLC resulting in the formation of PIP, phosphotidylinositol 4-phosphate. The primary substrate for PLD and PLA.sub.2 is phosphatidylcholine (PC), a relatively ubiquitous constituent of cell membranes. The activity of cytosolic PLA.sub.2 on PC liberates arachidonic acid, a precursor for the biosynthesis of prostaglandins and leukotrienes and possible intracellular secondary messenger. PLD, on the other hand, catalyzes the hydrolytic cleavage of the terminal phosphate diester bond of glycerophospholipids at the P-O position. PLD activity was originally discovered in plants and only relatively recently discovered in mammalian tissues. PLD has been the focus of recent attention due to the discovery of its activation by fMetLeuPhe in neutrophils. PLD activity has been detected in membranes and in cytosol. Although a 30 kD (kilodalton) and an 80 kD activity have been detected, it has been suggested that these molecular masses represented a single enzyme with varying extents of aggregation. See Cockcroft, Biochimica et Biophysica Acta 1113: 135-160 (1992). One PLD has been isolated, cloned and partially characterized. See Hammond, J. Biol. Chem. 270:29640-43 (1995). Biological characterization of PLD1 revealed that it could be activated by a variety of G-protein regulators, specifically PKC (protein kinase C ), ADP-ribosylation factor (ARF), RhoA, Rac1 and cdc-42, either individually or together in a synergistic manner, suggesting that a single PLD participates in regulated secretion in coordination with ARF and in propagating signal transduction responses through interaction with PKC, PhoA and Rac1. Nonetheless, PKC-independent PLD activation has been associated with Src and Ras oncogenic transformation, leaving open the possibility that additional PLDs might exist. See Jiang, Mol. and Cell. Biol. 14:3676 (1994) and Morris, Trends in Pharmacological Sciences 17: 182-85(1996). The difficulty may arise at least in part from the fact that in the phospholipase family enzymes may or may not be activated by, and catalyze, multiple substances, making sorting, tracking and identification by functional activities impractical.
There exists a need in the art for the identification and isolation of phospholipase enzymes. Without such identification and isolation, there is no practical way to develop assays for testing modulation of enzymatic activity. The availability of such assays provides a powerful tool for the discovery of modulators of phospholipase activity. Such modulators would be excellent candidates for therapeutics for the treatment of diseases and conditions involving pathological mitogenic activity or inflammation.